Liquid phase chromatography and extraction techniques are dependent upon the interaction of a region bonded to a solid support, such as silica, with a solvent environment containing the solute or solutes of interest. In such applications, the solid support is stationary relative to the liquid or solvent phase. The xe2x80x9cinteracting regionxe2x80x9d where the region bonded to the solid support and the solvent environment interact is commonly known as the xe2x80x9cbonded phasexe2x80x9d or xe2x80x9cinterphase.xe2x80x9d A general overview of technology relating to chromatographic applications may be found in, for example, C. Horvath, Silylated Surfaces (1980).
A useful, successful bonded phase is characterized by reproducible, non-binding interactions with solutes in the mobile phase under a variety of chromatographic operating conditions, including conditions of temperature and pressure, and conditions relating to the nature of the solvent used and the stability of the interaction over a period of time.
Liquid phase chromatography has numerous industrial, clinical, and research applications. Recently, there has been much interest in liquid chromatographic separations of polar analytes, such as carboxylic acids, nucleic acids, and other polar molecules. To achieve adequate retention and separation of many of these polar analyte compounds on alkyl bonded phase supports, it is necessary to employ straight aqueous mobile phases.
Use of conventional bonded phases such as the widely used octadecyl (xe2x80x9cC18xe2x80x9d) phase while employing straight aqueous mobile phases results in separations and analyte retentions that are initially satisfactory. However, after a few hours of contact with the straight aqueous mobile phase, the conventional bonded phase column typically exhibits loss of separation and of analyte retentions. This significantly reduces the accuracy and efficiency of chromatographic analysis.
It is conventionally considered that these observed phenomena are attributable to a xe2x80x9cfoldingxe2x80x9d process of the C18 ligand in the straight aqueous environment, sometimes referred to as xe2x80x9cphase collapse.xe2x80x9d A schematic representation of the xe2x80x9cfoldingxe2x80x9d phenomenon of the C18 ligand is illustrated in FIG. 1. It is believed that the folding phenomenon occurs because the straight alkyl C18 chain is at a higher thermodynamic energy state in the straight aqueous environment relative to it energy state in an aqueous/organic environment (e.g., water/acetonitrile). When the alkyl ligand undergoes folding, the hydrophobic region available for interaction with the polar analytes decreases in size, and a commensurate loss of analyte retention is observed. It is conventional practice to xe2x80x9cre-extendxe2x80x9d the folded alkyl chains by passing a small amount of a polar solvent, such as methanol, through the C18 column. The analyte retentions will then return to their initial values after a short equilibration with the straight aqueous mobile phase. However, this remedy is transient; after a period of time, the analyte retentions will again decrease as a function of the C18 chain folding and the re-extending process must be repeated.
Prior art efforts to eliminate or minimize the previously described folding phenomenon with C18 columns employing straight aqueous mobile phases include use, in the bonded phase, of molecules employing xe2x80x9cpolar embedded groupsxe2x80x9d at some position in the alkyl chain. For example, a schematic representation of a polar embedded group in an alkyl chain is provided in FIG. 2. Groups conventionally considered useful as polar embedded groups include amides, esters, and ethers. The polar embedded groups serve to facilitate the wetting of the hydrocarbon (C18) chain and thereby minimize the folding phenomenon. A disadvantage of this approach is that the system is often unstable when mobile phases of low pH values (e.g., pH 2-3) are used, and in some cases, the system exhibits unfavorable analyte selectivities.
Additionally, branched alkylsilanes having a branched hydrocarbon backbone with branched alkylsilane moieties extending asymmetrically from the backbone also exhibit minimal phase collapse when used as a bonded phase in chromatographic and separation applications. Such branched alkylsilanes are disclosed in U.S. Pat. No. 5,874,603, issued Feb. 23, 1999.
Accordingly, there is a need in the art for a material for forming an improved bonded phase that exhibits the increased bonded phase interaction of C18 and good retention characteristics over time, while minimizing the effect of chain folding (phase collapse) in straight aqueous mobile phases. Further, there is a need for a simple synthesis reaction for forming a trialkylsilane having a relatively long chain length with a high level of purity for use in a bonded phase.
The invention is a trialkylsilane comprising a hydrocarbon backbone including one to ten carbon atoms; a terminal trialkylsilyl moiety on the backbone having at least two alkyl groups independently comprising at least six carbon atoms; and a terminal silyl moiety on the backbone that has at least one hydrolyzable group bound to a silicon of the silyl moiety. At least one of the alkyl groups of the trialkylsilyl moiety may be substituted, preferable with at least one halogen. Most preferably, at least one of the alkyl groups of the trialkylsilyl moiety is a perfluoroalkyl group. Additionally, the invention is a trialkylsilane represented by the formula (I); 
wherein R1 is a linear or branched alkyl chain of one to four carbon atoms; R2 is independently selected from a substituted, unsubstituted linear or branched alkyl chain having m carbons, where m is an integer such that 6xe2x89xa6mxe2x89xa618; R3 is independently a hydrogen atom, or a branched or linear, substituted or unsubstituted alkyl group; p is an integer of one to ten; R4 is a linear or branched alkyl group of one to four carbon atoms or an aryl group; n is an integer of zero to two; and x is a hydrolyzable group.
The invention also encompasses a treated substrate for use in chromatographic applications. The treated substrate includes a substrate and the trialkylsilane of the invention.
Further contemplated within the scope of the invention is a method for preparing the trialkylsilane for use in chromatographic applications. The method includes preparing a disubstituted vinylsilane by reacting an organomagesium reagent and an alkyl vinylfunctionalsilane or an aryl vinylfunctionalsilane to obtain a disubstituted alkylvinylsilane or a disubstituted arylvinylsilane; reacting the disubstituted vinylsilane of step (a) in a monomeric silane containing a silicon-hydrogen bond in the presence of a metallic catalyst. The monomeric silane is added to the vinyl group of the vinylsilane, thereby binding the silicon of the monomeric silane to the terminal carbon of the vinyl group and forming a trialkylsilane having a terminal trialkylsilyl moiety wherein at least two alkyl groups of the trialkylsilyl moiety independently comprise at least six carbon atoms and the least two alkyl groups extend from the silicon of the prepared vinylsilane.
Also described herein is a method for forming a bonded phase for use in chromatographic applications. The method includes forming a trialkylsilane which comprises a hydrocarbon backbone having one to ten carbon atoms, a terminal trialkylsilyl moiety (having at least two alkyl groups independently comprising six to eighteen carbon atoms) on the hydrocarbon backbone, a terminal silyl moiety on the backbone wherein the silicon of the silyl moiety is capable of being bound to a substrate, and reacting the trialkylsilane with a substrate comprising silicon in the presence of a hydroxy-containing compound to form an Sixe2x80x94Oxe2x80x94Si bond between the silicon of the silyl moiety and the silicon of the substrate, such that the at least two alkyl groups of the trialkylsilyl moiety extend to provide a bonded phase useful for reproducible molecular interaction.
The invention also includes a column for use in chromatographic applications. The column includes a treated substrate that contains the trialkylsilane of the invention.